Thermal spraying, also known as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form in a carrier gas. Such powders are typically comprised of small particles, e.g., between 100 mesh U. S. Standard screen size (149 microns) and about 2 microns. Alternatively, wire is used as the feed material.
A thermal spray gun normally utilizes a combustion or plasma flame to produce the heat for melting of the powder particles. Other heating means may be used as well, such as electric arcs, resistance heaters or induction heaters, and these may be used alone or in combination with other forms of heaters.
A particular challenge is spraying on the inside surfaces of confined areas such as in holes, pipes and the like. The guns normally spray forwardly with a spray distance of at least several centimeters, and an ordinary spray gun is at least 15 cm long, restricting the ability to spray sideways in a small hole. In the past various adaptations have been made for coating inside surfaces. In the simplest case only the nozzle is turned sideways on the end of an extension, as disclosed for a powder flame spray gun in U.S. Pat. No. 3,171,599 (Rotolico). This is not possible for a wire spray gun since the extension must accommodate the relatively stiff wire. Therefore other deflectors were devised, including blasting the melting wire tip with air from sideways (U.S. Pat. No. 3,136,484, Dittrich), curving the air cap (U.S. Pat. No. 3,122,321, Wilson et al), and a combination of these (U.S. Pat. No. 3,056,558, Gilliland et al). In a plasma spray gun a double angle nozzle has been used (U.S. Pat. No. 3,707,615, Rotolico et al).
None of the aforementioned approaches has been adaptable to provide an extension for a recently developed high velocity thermal spray gun of the type disclosed in U.S. Pat. No. 4,865,252 of the present assignee. The complexity of the high velocity gas head is not readily miniaturizable to turn sideways, the very high velocity flame spray stream cannot be deflected sufficiently, and a conventional curved gas cap is susceptible of erosion and powder buildup.